Electrical pulse tranformation using optical delay lines

ABSTRACT

A recirculating optical delay line  30  has a laser  32  controlled by wavelength control  33  so as to vary the wavelength of radiation over time to provide a first input  34  to an optical modulator  35 . The modulator  35  modulates the intensity of the first input  34  with a pulsed electromagnetic frequency signal  36  to produce a pulsed modulated optical signal  39 . The signal  39  passes through an optical coupler  40  into a delay loop  41  having a delay fibre  44  arranged to delay the signal  39  for a predetermined duration. The wavelength control  33  is arranged to vary the first input  35  so as to ensure that overlapping pulses of the signal  39  in the delay loop  41  are at different wavelengths, thereby inhibiting optical mixing effects between the overlapping pulses.

[0001] The present invention relates to an optical delay line and to amethod of generating a delayed optical signal.

[0002] Optical fibre technology is gradually making an impact onelectronic systems through their ability to implement various signalprocessing functions, for example producing multiple delays andmicrowave filtering. Optical fibres also offer a low loss, compactsolution to the generation of long delays for optical signals. The lowdispersion properties of optical fibres effectively provide modulationfrequency independent delay lines that minimise degradation of an inputradio frequency signal.

[0003] Currently such delays are introduced using recirculating opticaldelay lines which offer a number of advantages over alternative serialor parallel type delay line architectures and provide a low costsolution for the generation of a range of delays of a pulsed signal.

[0004] From FIG. 1, a prior art recirculating optical delay line 10comprises a fixed wavelength carrier wave optical source II providing afirst input 12 to an external optical modulator 13 arranged to modulatethe light from the optical source 11 with a pulsed radio frequencysignal 14 which passes through a radio frequency amplifier 15 to act asa second input 16 to the external optical modulator 13.

[0005] A modulated optical signal 17 is generated by the externaloptical modulator 13 which then passes through a two-by-two opticalcoupler 18 that is arranged to allow 50% of the modulated signal 17 toenter a delay loop 19 and the other 50% of the modulated signal 17 tobypass the delay loop 19 and to proceed to an output of the optionalcoupler 18. The delay loop 19 comprises an optical amplifier 20 inseries with a band pass optical filter 21 and a delay fibre 22. It willbe understood that the modulated signal 17 is a series of radiofrequency modulated pulses having a pulse length determined by thepulsed radio frequency signal 14.

[0006] In this delay line 10 the modulated signal 17 enters the delayloop 19 through the two-by-two optical coupler 18 and circulates throughthe amplifier 20, filter 21 and delay fibre 22 to achieve the desireddelay duration of each pulse of the modulated signal 17. It is importantto note that the pulse length of each pulse of the modulated signal 17must be equal to or less than the overall delay duration of the delayloop 19 in order to prevent coherent optical mixing affects betweenoverlapping sections of the same pulse of the modulated optical signal17. The coupler 18 is also arranged to extract delayed optical pulses 23from the delay loop 19 after each circulation of a pulse of themodulated signal 17 around the delay loop 19. Each delayed optical pulse23 is detected by a photodiode 24 which serves to convert each delayedoptical pulse 23 into an electrical signal 25 which passes through aradio frequency amplifier 26 so as to produce a delayed pulsed radiofrequency output 27.

[0007] However, prior art recirculating optical delay lines 10 areconstrained by the requirement that the pulses of the radio frequencymodulated signal 17 must have a shorter duration than the recirculatingduration of the pulse around the delay loop 19 in order to preventcoherent optical mixing effects between overlapping sections of the samepulse of the modulated signal 23 when they are detected on thephotodiode 24. In some applications the pulse duration of the modulatedsignal 17 may be unknown or uncontrolled.

[0008] EP-A2-0,997,751 addresses the problem of coherent mixing effects,but for overlapping sections of successive pulses rather than foroverlapping sections of the same pulse as is the problem here. Theoverlap between successive pulses is caused by dispersion within opticalfibres causing pulse broadening to the extent that the trailing edge ofone pulse overlaps with the leading edge of the successive pulse. Thisproblem is addressed by applying a desired phase response to pulses.

[0009] The article “Continuously Variable True Time-Delay Optical Feederfor Phased-Array Antenna Employing Chirped Fiber Gratings” by Corral etal. (IEEE Transactions on Microwave Theory and Techniques, vol. 45,pages 1531-1536), EP-A1-0,392,416 and U.S. Pat. No. 5,210,807 discloseoptical delay lines employing wavelength-tuneable lasers as theiroptical sources, although none of these documents addresses the problemof coherent optical mixing effects due to overlapping sections ofoptical pulses. Instead, all three documents address a common problem ofobtaining a variable delay time from a single delay line.

[0010] In the case of the IEEE Article and U.S. Pat. No. 5,210,807,variable delay times are achieved by using a series of Bragg gratings atdifferent positions along the delay line, such that different pathlengths are obtained by reflecting selectively from different Bragggratings. The Bragg gratings have different reflecting wavelengths, sothat a desired reflection point along the delay line can be selected byinjecting light with the appropriate wavelength. This is done by tuninga laser to the appropriate wavelength, this wavelength being applied tothe entire pulse such that a uniform time delay is obtained for theentire pulse.

[0011] In the case of EP-A-0,392,416, variable delay times are achievedby using a highly-dispersive optical fibre, such that the time taken fora pulse of light to propagate through the delay line varies appreciablywith the wavelength of light. A laser is tuned to the appropriatewavelength, the same wavelength being used throughout an entire pulse sothat a uniform time delay is obtained for the entire pulse.

[0012] It is an object of the present invention to obviate or mitigatethe problems associated with the prior art, especially to inhibitcoherent optical mixing of pulses of light in a delay line.

[0013] According to a first aspect of the invention an optical delayline, comprising an optical source arranged to generate intensitymodulated pulses of light, a delay fibre arranged to carry the pulses oflight, an optical coupler arranged to allow the pulses of light to enterand to exit the delay fibre and characterised by an optical sourcecontroller arranged to vary the wavelength of each pulse of light suchthat each successive portion of each pulse of light within the delayfibre is segregated by wavelength.

[0014] In this manner, the optical source controller ensures thatoverlapping pulses of intensity modulated light within the optical delayline are at different optical wavelengths and hence coherent opticalmixing effects between the overlapping pulses of light is inhibited.Therefore, input pulses of light to the delay fibre can have a longerduration than the delay fibre delay duration without distortion of thepulses of light due to coherent optical mixing effects betweenoverlapping pulses of light and the optical delay line can be optimisedfor shorter delay times.

[0015] In one embodiment of the invention a delayed pulse combiner maybe arranged to construct a delayed electrical signal from the delayedpulses of light after they have exited the delay fibre in accordancewith the wavelength of the portions of each delayed pulse of light. Thedelayed pulse combiner may comprise one or more Bragg fibre gratingsarranged to segregate the portions of the pulses of light after theyhave exited the delay fibre in accordance with their wavelength. Atleast one of the Bragg fibre grating may have an associated photodiodearranged to convert impinging portions of the pulses of light into anelectrical sub-signal. An electrical combiner may be arranged toconstruct the delayed electrical signal from the electrical sub-signalsproduced by photodiode associated with each Bragg grating. In thismanner, pulses of light of a given wavelength are detected prior tobeing combined so as to reconstruct the delayed electrical signal. Theoptical coupler may be a 2 to 2 optical coupler.

[0016] In another embodiment of the invention a tuneable bandpassoptical filter may be arranged to construct a delayed electrical signalfrom the portions of the delayed pulses of light after they have exitedthe delay fibre in accordance with the wavelength of each delayed pulseof light. The tuneable bandpass optical filter may be arranged to allowportions of the pulses of light to exit the delay fibre in accordancewith their wavelength. The tuneable bandpass optical filter may becontrolled by the optical source controller so as to vary the wavelengthof the tuneable bandpass optical filter in time with the variation inthe wavelength of the portions of the pulses of light. Preferably, theoptical coupler may comprise a 2 by 1 optical coupler and an opticalcirculator.

[0017] An optical modulator may be arranged to generate the pulses oflight by modulating a light source with a pulsed electromagneticfrequency input. For example, the electromagnetic frequency input may bea pulsed radio frequency input, in this matter, a delayed pulsed radiofrequency output from the optical delay line is achieved.

[0018] The optical source may be a distributed feedback semiconductorlaser.

[0019] According to another aspect of the invention, there is provided amethod of generating a delayed electrical signal comprising generatingmodulated pulses of light, passing the pulses of light through a delayfibre, constructing the delayed electrical signal from the electricalsub-signals, and charactensed by varying the wavelength of the pulses oflight with respect to time such that successive portions of each pulseof light within the delay fibre are segregated by wavelength, and byconverting delayed pulses of light to electrical sub-signals after theyhave exited the delay fibre according to the wavelength of the portionsof each delayed pulse of light.

[0020] The invention will now be described, by way of example only, withreference to the accompanying drawings, in which:

[0021]FIG. 1 illustrates a prior art recirculating optical delay line;

[0022]FIG. 2 illustrates an embodiment of a recirculating optical delayline according to the present invention;

[0023]FIG. 3 illustrates the relationship between an input pulse of agiven duration and a recirculating duration of an optical delay lineaccording to the invention;

[0024]FIG. 4 illustrates the relationship between laser wavelength andduration of an optical delay line according to one embodiment of theinvention;

[0025]FIG. 5 illustrates the relationship between laser wavelength andduration of an optical delay line according to an alternative embodimentof the invention, and

[0026]FIG. 6 illustrates an alternative embodiment of a recirculatingoptical delay line according to the present invention.

[0027] Referring to FIG. 2, a recirculating optical delay line 30comprises a variable wavelength carrier wave optical source 31comprising a laser 32 having a wavelength control 33 to vary thewavelength of radiation generated by the laser 32 with respect to timeso as to provide a first input 34 to an external optical modulator 35.The external optical modulator 35 is arranged to modulate the intensityof the first input 34 from the optical source 31 with a pulsedelectromagnetic frequency signal 36, for example a pulsed radiofrequency signal, which passes through an amplifier 37 to act as asecond input 38 to the external optical modulator 35.

[0028] A modulated optical signal 39 is generated by the externaloptical modulator 35 which then passes through a two-by-two opticalcoupler 40 that is arranged to allow 50% of the modulated signal 39 toenter a delay loop 41 and the other 50% of the modulated signal 17 tobypass the delay loop 41 and proceed to an output of the optical coupler40. The delay loop 41 comprises an optical amplifier 42 in series with abandpass optical filter 43 and a delay fibre 44. It will be understoodthat the modulated signal 39 comprises a series of intensity modulatedoptical pulses having a pulse length determined by the pulsed signal 36and an intervening period having a reduced intensity, that is acontinuous signal during the interpulse period.

[0029] The optical coupler 40 is also arranged to extract delayedoptical pulses 45 from the delay loop 41 after each circulation of apulse of the modulated signal 39 has circulated around the delay loop41.

[0030] The optical pulses 45 pass to a delayed signal combiner 46arranged to construct a delayed electrical signal 47 from the delayedoptical pulses 45 extracted from the delay loop 41 in accordance withthe wavelength of each delayed optical pulse 45. The operation of thedelayed signal combiner 46 is described in greater detail below. Eachdelayed electrical signal 47 is amplified in a signal amplifier 48 so asto produce a delayed pulsed output 49.

[0031] The delayed signal combiner 46 comprises a number of sub-units 50arranged in series with one another, each comprising a opticalcirculator 51 arranged to allow the delayed optical pulses 45 to pass toa Bragg fibre grating 52 arranged to either reflect delayed opticalpulses 45 having a wavelength corresponding to that written into theBragg fibre grating 52 or to allow delayed optical pulses 45 of analternative wavelength to pass onto the next sub-unit 50. It will beunderstood that the delayed optical pulses 45 will pass through theoptical circulators 51 of the sub-units 50 until they are reflected by aBragg fibre grating 52 having a corresponding wavelength. A reflecteddelayed optical pulse 45 will return to the optical circulator 51associated with the Bragg fibre grating 52 from which it was reflected,where it will then be diverted by the optical circulator 51 down anassociated arm 53 to a photodiode 54 which serves to convert the delayedoptical signal 45 diverted to that arm 53 into an electrical signal 55which then passes through a signal amplifier 56 so as to produce adelayed electrical sub-signal 57. An electrical combiner 58 serves tocombine the electrical sub-signals from each arm 53 so as to produce asingle output, that is electrical signal 47. The delayed signal combiner46 ensures that each sub-signal 57 of a different wavelength is detectedseparately before being summed coherently at the electrical combiner 58.

[0032]FIG. 3 indicates the relationship between the pulse length T ofthe pulsed signal 36, and the delay recirculation time T₁ of a modulatedsignal 39 through the delay loop 41 described with reference to FIG. 2.It will be noted that the recirculation optical delay line 30 has beenarranged to accommodate pulsed signals 36 whose pulse length is up tofour times the duration provided by the recirculating delay line 41.That is in FIG. 3 the pulse length T is over three times the length ofthe duration T₁ provided by the delay line. It will be understood, thatmore sub-units 52 can be added to the delay compensator 46 if a longerpulsed signal 36 pulse length is required to be accommodated by thedelay loop 41. From FIG. 3, it will be noted that the input pulse lengthT does not need to be an integer multiple of T₁. This is indicated bythe time period of pulse length T₂, where T₂≦T₁.

[0033] The laser 32 of FIG. 2 can be a distributed feedbacksemi-conductor laser having a line width of between 1 to 4 MHz and itswavelength can be adjusted by controlling a combination of its inputbias current (typically 1.1 GHz per mA) and its temperature (typically0.1 nm per C.° which is approximately equivalent to 12.5 GHz per C.°).The bias current provides a fast wavelength control response while thetemperature control of the laser gives a large tuning range at a slowerrate.

[0034]FIG. 4 indicates the relationship between laser wavelength shownalong the ordinate 60 and the delay line recirculating time T₁ given astime t along the abscissa 61. It is the objective of the variation inlaser wavelength, illustrated as graph line 63, to ensure that delayedoptical pulses within a delay loop at substantially the same timeoperate at different wavelengths. The wavelength variation betweenrecirculations can be controlled, as indicated in FIG. 4, using a linercontrol waveform, that is graph line 63. Alternatively, as indicated inFIG. 5 in which like references have been used to indicate similarintegers to those shown in FIG. 4 the wavelength variation can becontrolled using a step wavelength control waveform, illustrated asgraph line 64. An advantage of the stepped wavelength control waveformis that the laser is not required to sweep back to an initial startingwavelength at some point during the wavelength variation.

[0035] For example, if the line width of the laser is 4 MHz, thelinewidth will broaden to approximately 0.46 nm (that is equivalent to57.4 GHz) at a level of approximately 30 dBc. For recirculation delaytimes T₁ of 10 microseconds, this requires a wavelength tuning rate of0.046 nm per microsecond (that is equivalent 5.7 GHz per microsecond).It will be understood that different laser wavelength tuning rates willbe required for use with alternative recirculating delay times T₁. The0.046 nm per microsecond wavelength tuning rate will require a lasertuning control algorithm which combines both laser temperature and biascurrent control, as is known from the prior art.

[0036] Tuneable semi-conductor laser diodes can offer a widerelectronically controlled tuning range and may be suitable forapplication requiring a larger tuning range.

[0037] In FIG. 6, in which like references have been used to indicatesimilar integers to those shown in FIG. 2, the two by one opticalcoupler 70 allows the modulated signal 39 to enter the delay loop 41which also includes, in series with the optical amplifier 42, thebandpass filter 43 and the delay fibre 44, an optical circulator 71which allows delayed optical pulses 45 to pass to an optical tuneablebandpass filter 72 that allows a delayed optical pulse 45 of a givenwavelength to pass to a single photodiode 73. The optical tuneablebandpass filter 72 can be controlled by a wavelength control 74 arrangedto allow delayed optical pulses 45 of the correct wavelength to passthrough the optical tuneable bandpass filter 72 or the wavelengthcontrol 74 can be substantially the same as wavelength control 33 to thelaser 32. Delayed optical pulses 45 which do not pass through theoptical tuneable bandpass filter 72 are reflected and recirculate aroundthe delay loop 41 until such time as the wavelength control 74 allowsthe optical tuneable bandpass filter 72 to pass delayed optical pulse 45of that given wavelength. It will be understood that it is the controlof the optical tuneable bandpass filter 72 which allows the constructionof the delayed electrical signal 47.

[0038] The tuneable bandpass filter 72 of FIG. 6 enables a singlerecirculated pulse 45 to be selected from multiple recirculated pulseswithin the delay loop 41 whereas the optical delay line 30, asillustrated in FIG. 2, outputs all delayed pulses in the delay loop 41.

1. An optical delay line (30), comprising an optical source (31)arranged to generate intensity modulated pulses of light, a delay fibre(41) arranged to carry the pulses of light, an optical coupler (40; 70)arranged to allow the pulses of light to enter and to exit the delayfibre, and characterised by an optical source controller arranged tovary the wavelength of each pulse of light such that each successiveportion of each pulse of light within the delay fibre is segregated bywavelength.
 2. An optical delay line, as in claim 1, wherein a firstcombiner (56; 48) is arranged to construct a delayed electrical signalfrom the delayed pulses of light after they have exited the delay fibrein accordance with the wavelength of the portions of each delayed pulseof light.
 3. An optical delay line, as in claim 2, wherein the firstcombiner comprises one or more Bragg fibre gratings (52) arranged tosegregate the portions of the pulses of light after they have exited thedelay fibre in accordance with their wavelength.
 4. An optical delayline, as in claim 3, wherein at least one of the Bragg fibre grating hasan associated photodiode (56) arranged to convert impinging portions ofthe pulses of light into an electrical sub-signal.
 5. An optical delayline, as in claim 4, wherein a second combiner (48) is arranged toconstruct the delayed electrical signal from the electrical sub-signalsproduced by each photodiode associated with each Bragg grating.
 6. Anoptical delay line, as in any preceding claim, wherein the opticalcoupler is a 2 by 2 optical coupler.
 7. An optical delay line, as inclaim 1, wherein a tuneable bandpass optical filter (72) is arranged toconstruct a delayed electrical signal from the delayed pulses of lightafter they have exited the delay fibre in accordance with the wavelengththe portions of each delayed pulse of light.
 8. An optical delay line,as in claim 7, wherein the tuneable bandpass optical filter is arrangedto allow the portions of the pulses of light to exit the delay fibre inaccordance with their wavelength.
 9. An optical delay line, as in claims7 or 8, wherein the tuneable bandpass optical filter is controlled bythe optical source controller so as to vary the wavelength of thetuneable bandpass optical filter in time with the variation in thewavelength of the portions of the pulses of light.
 10. An optical delayline, as in any of claims 7 to 9, wherein the optical coupler comprisesa 2 by 1 optical coupler (70) and an optical circulator (71).
 11. Anoptical delay line, as in any preceding claim, wherein an opticalmodulator is arranged to generate the pulses of light by modulating alight source with a pulsed electromagnetic frequency input (38).
 12. Anoptical delay line, as in any preceding claims, wherein the opticalsource includes a distributed feedback semiconductor laser.
 13. Anoptical delay line substantially as illustrated in and/or described withreference to FIGS. 2 to 6 of the accompanying drawings.
 14. A method ofgenerating a delayed electrical signal, including generating modulatedpulses of light, passing the pulses of light through a delay fibre (41),constructing the delayed electrical signal from the electricalsub-signals, and characterised by varying the wavelength of the pulsesof light with respect to time, such that each successive portion of eachpulse of light within the delay fibre is segregated by wavelength, andconverting the delayed pulses of light into electrical sub-signals afterthey have exited the delay fibre and according to the wavelength of theportions of each delayed pulse of light.
 15. A method of generating adelayed optical signal substantially as illustrated in and/or describedwith reference to FIGS. 2 to 6 of the accompanying drawings.